Jamie, We had to modify all microHAM interface cables for the K3 to move the PTT ground from Pin 8 to the shell of the Foster (mic) jack to avoid the "Pin 1 problem" caused by the common RF choke (L7) on the front panel board.
I suspect your multi-switcher also connects PTT to pin 2/8, mic to 1/7 and a shield between the chassis of both units. Moving the PTT ground from pin 8 to the shell of the Foster plug (or connecting PTT ground to the chassis in the multi-switcher and leaving pin 8 open) should eliminate the problem completely. As W9AC reports, replacing L7 with a jumper seems to be the best solution but it requires disassembling the front panel of the radio. A good "work around" is to install a jumper between pin 8 and the shell of your mic connector. 73, ... Joe, W4TV > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of James > C. Hall, MD > Sent: Saturday, November 01, 2008 1:13 PM > To: 'Dr. James C. Garland'; [email protected] > Cc: 'W. Douglas McDowell' > Subject: RE: [Elecraft] K3 RF Feedback Problem > > > Hi Jim: > > I'm wondering what external station controller you are using. > I'm using a NCS Multi-Switcher and I ran into very similar > issues. I certainly felt (as did Doug at NCS) that it was a > ground loop problem. I used transformer isolated mike cables > and found that it was NOT a ground problem. I can give you my > long details, but the bottom line is that the mike gain in > the Multi-Switcher (internal) needed to be driven a bit > higher than normal, and the mike gain on the K3 need to be > quite low (5 or 6) to avoid the 'growl' effect. I'm > forwarding this to Doug at NCS as he is quite interested in > what's going on that might be causing this. > > 73, Jamie > WB4YDL > > > > > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Dr. > James C. Garland > Sent: Saturday, November 01, 2008 8:57 AM > To: [email protected] > Subject: [Elecraft] K3 RF Feedback Problem > > Gang, > > I'm pulling my hair out over an RF feedback problem with my > K3/100, and wonder if anybody has any ideas? I use an > external station controller that switches key, microphone, > CAT, line in/out, etc. between different rigs. The > microphone audio connects to the K3's rear panel microphone > jack and when the K3's power exceeds about 15W (in SSB), the > audio becomes greatly istorted. It does this when all the > other cables to the K3 (except the microphone and power cord) > are unplugged. It also does it when the audio is fed to the > K3 via the rear Line In jack. Everything in my station is > well-grounded. Here are some things I've noticed: > > 1. The feedback still occurs when a military-type completely > shielded 100W dummy load is screwed directly to the K3's > antenna port. This suggests the RF from the K3 is getting > into the audio line from some other source than radiation > from the coax cables in the station, most likely the 12V > power cord. To support this conjecture, I also notice that > the feedback threshold (15W) is not changed when the K3 > drives a linear amplifier. > > 2. I normally use a 30A Kepco 12V power supply to run the K3 > and my other station accessories. The RF feedback problem is > improved but not entirely eliminated if I use a separate 12V > supply for the K3. A common mode choke on the 12V line > doesn't appear to make any difference. > > 3. I can see some modulation-induced fluctuationss on the 12V > line with an oscilloscope that increase with the K3's power > setting. I haven't looked at these fluctuations closely > enough yet to know whether it's RF noise or just > audio-frequency fluctuations caused by the K3's modulating > current draw from the power supply. > > To summarize, my conclusions so far are that (1) the 12V > power cord is a source of RF leakage from the K3, even > though the K3 has a filter at its 12V power connector to > minimize this leakage; and (2) The audio/DSP circuitry in the > K3 is very sensitive to RF - much more so than other > transceivers in my station. (I've not had this problem with > other rigs.) > > Because of this RF sensitivity, one evidently has to be very > careful when hooking accessories to the K3. The front panel > microphone jack is (to me) wired in a curious way, with the > Mic ground and PTT ground (shield) connections floating above > the K3's chassis with a 100uH rf choke. Similarly, both the > "hot" and "shield" side of the rear panel mic input jack has > series 100uH rf chokes, which isolate the input from chassis ground. > > The problem with this arrangement is that most accessories > that would connect to the rear panel audio input jacks have > single-ended outputs, with a shielded cable that is directly > tied to chassis ground at the accessory end of the cable. > Thus, unless one is very careful, it is easy to have a > situtation where the signal grounds in the audio circuitry of > the K3 can fluctuate with respect to the K3's chassis ground, > and this may be the source of the RF feedback sensitivity. > > Unfortunately, I can't figure out the detailed mechanism for > the feedback closely enough to figure out a solution. Foir > example, I don't know whether it's better to leave the minus > side of my 12V power supply floating, or to tie it to the > chassis gound.. And even if I left it floating, it would be > tied to the chassis anyway by the other accessories hooked to > it, and this might cause more problems than grounding it at > the power supply terminal. Somehow, it seems like a wiring > change in the ground configuration of the K3's audio circutis > to improve RF isolation may be needed to lick the problem completely. > > 73, > > Jim Garland W8ZR > > > _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
